The present invention relates to methods for coding and decoding an image by dividing the image into blocks, and to an image coder and image decoder employing these methods.
Data compression is a field of intense interest, because it is an enabling technology for multimedia. Image data compression methods have been standardized by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T), and by the International Organization for Standardization and the International Electrotechnical Commission (ISO/IEC). For still images, the method advocated by the Joint Photographic Experts Group (JPEG) has been adopted in ITU-T recommendations T.81 and T.83 and ISO/IEC standard 10918. For moving images, methods advocated by the Moving Picture Experts Group (MPEG) have been adopted in ISO/IEC standard 11172 (MPEG1) and ISO/IEC standard 13818 (MPEG2).
The JPEG and MPEG methods divide an image into blocks, and compress each block separately by a combination of the discrete cosine transform, quantization, and entropy coding. These techniques require complex calculations and much memory access, however, so their speed is limited, and an image coding and decoding apparatus (codec) employing the JPEG and MPEG methods is comparatively large and expensive.
In addition, the JPEG and MPEG methods are not directly applicable when the decoded image is reduced or enlarged by changing the image resolution; that is, by changing the number of picture elements (pixels) in the image. Resolution conversion has to be performed as a separate operation, following image decoding.